1. Object of the Invention
This invention relates to an improved seismic energy source which generates low-frequency signals used in searching for petroleum under a waterbody. In particular, it relates to a seismic source having a number of closed flexible sleeves which transmit a varying frequency modulated signal into the water. A relatively low pressure hydraulic source keeps the sleeves inflated and oscillates the sleeve's compliant walls at the desired frequency.
2. Field of the Invention
The ever more difficult search for petroleum has led to exploration in areas which were through by many just a short time ago to be incapable of producing petroleum at an economically feasible price. The rising price of petroleum coupled with its relative domestic scarcity has made acceptable the costs associated with production in such forbidding regions as Alaska and the North Sea, as well as in a number of North American offshore areas.
Of the many methods used in the prospecting for subsea petroleum, few have attained as widespread an acceptance as has the use of towable marine seismic sources.
The theory of operation in using acoustic sources to search for petroleum is quite simple. An acoustic signal is introduced into the water body from an acoustic source towed behind a ship. The acoustic wave propagates down through the water, across the water-floor interface, and into the subfloor geologic formations. The resultant echoes are, so some extent, reflected back across the same path to an array of geophones waiting near the water's surface. Analysis of the signals produced by the geophones can provide some instruction concerning the structure of the subfloor geological formations and attendant petroleum accumulation within those formations.
The term "water" as used herein is meant to include swampwater, mud, marshwater, seawater or any other liquid containing sufficient water to enable operation of the invention.
There are a number of different methods for producing an acoustic pulse. For instance, the earliest attempts entailed the use of solid explosives. This method produces a strong acoustic wave which, accordingly, achieves substantial penetration into subfloor geologic formations and a strong return echo. Solid explosives possess certain obvious drawbacks: they are dangerous to store, handle, and use. When set off in open water, they kill marine life. In a crowded area such as a harbor, they cannot be used at all. Solid explosives are orders of magnitude more expensive to use, on a per-shot basis, than are most other acoustic sources. Modification of the acoustic signature to achieve an acceptable spectrum distribution is most difficult.
Apparatus using explosive gas mixtures, e.g., propane and oxygen, to produce an acoustic signal in the form of a pulse or shock wave have gained wide acceptance. The two major types of explosive gas guns are: first, those which operate by exploding a combustible gas mixture behind a flexible membrane which is in turn in contact with the water; and, second, those which operate by allowing the abrupt bubble from the gas explosion to pass directly into the water. An example of the former apparatus can be found in U.S. Pat. No. 3,658,149; an example of the latter apparatus can be found in U.S. Pat. No. 4,193,472.
Devices using high pressure compressed gases to generate an acoustic pulse are also widely used by the industry. These apparatus, or guns, typically employ a gas-holding chamber which is first pressurized to a pre-set level and then is fired by allowing the pressurized gas to explosively exit the gun into the surrounding water. Examples of open-ported pressurized gas guns are found in U.S. Pat. No. 3,653,460, to Chelminski, and U.S. Pat. No. 4,141,431, to Baird.
The device of the present invention is a member of a class which generates a relatively low-power and low-frequency (10-100 Hz) acoustic signal which extends over a period of seconds. The transmitted signals are by design low-frequency to reduce attenuation losses in the reflected waves. The transducer in the disclosed device is an array of flexible sleeves in contact with the water and is driven by a modulated hydraulic actuator. Unlike the previously mentioned devices which emit but a single spike-like pulse and thereby provide a discreet echo at some readily determinable point in time, these devices often vary the frequency of the transmitted signal in some pre-set manner so that a unique instantaneous frequency in the reflected signal can be correlated with that frequency in the transmitted signal. The reason for varying a property such as frequency during a signal should be apparent. A spike-like pulse has but a single echo from a particular point below the ground. A single frequency signal that lasts several seconds would result in echoes carrying little or no useful information. It would be substantially impossible to determine whether the echo resulted from the beginning of the signal or its end. Consequently, the time for transit of the signal into the earth and return would be lost.
A signal used by the invention may have a duration of several seconds and linearly sweep the frequency range between two points, e.g. 10 Hz and 100 Hz, whereupon the signal ceases and restarts at 10 Hz. During each sweep, the instantaneous frequency of received signal can be correlated with high precision with that of the transmitted signal. Variations of the transmitted signal may be desirable. For instance, the swept signal need not be continuous throughout the sweep range. A non-continuous signal can solve a problem known as "ringing". This occurs when the distance between the water's surface and an acoustically hard bottom is some discreet multiple of an acoustic signal wavelength. A standing wave is created which, upon reception by the hydrophones, effectively overrides and obliterates any meaningful information carried in returning echoes having a different frequency. If the "ringing" frequency is, e.g., about 21 Hz, then an acoustic sweep ranging from 10 Hz to 100 Hz can be silenced between 17 Hz and 25 Hz. Other seismic sources which emanate "white noise", such as gas guns, cannot be so readily tuned and therefore may be completely ineffective in certain subsea areas.
It is not necessary that the signal be linearly increasing. It may be neither linear nor increasing just so long as the frequency of the echoed signal can be closely correlated as a function of time with the transmitted signal.
Another major problem associated with the low-power, low-frequency chirp seiemic sources is one of efficiency. It is difficult to design a hydraulically powered seismic source in which a significant proportion of its input power appears in the output acoustic signal. Consequently, a number of hydraulically powered seismic sources use a fairly high internal hydraulic pressure to assure sufficient and usable signal strength in the returning echo.
The device disclosed herein is intended to provide high efficiency by maximizing the area of the flexible transducer exposed to the water. By doing so, the seismic source also allows the use of a relatively low pressure hydraulic system.